The US (provides arms, training and military assistance to "moderate" groups)

Jordan (provides logistical support and training)

*The term "rebels" is used to describe a huge and diverse array of fighters, some of whom co-operate with jihadists like those from Hayat Tahrir al-Sham, an al-Qaeda-linked alliance. Different foreign states often back different rebel factions.

At this point, it's also worth noting that although the US supports some rebel groups, it had not intentionally attacked the Syrian government directly until Friday, when it fired missiles at an airbase used in an alleged chemical attack.

Wait, what? I've definitely heard about US strikes in Syria before.

Yes, you have. But those were targeting IS militants as part of an international campaign that began in September 2014.

Lots of other countries are taking part, including the UK, France, Jordan and several Gulf States.

Basically, IS exploited the war in Syria to carve out a large part of the country for the "caliphate" whose creation it proclaimed three years ago.

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IS militants were driven out of the city of al-Bab by Turkish-backed rebels in February

This is where the battle-lines get blurry. The Syrian government and Russia are also bombing IS, but separately to the US-led coalition.

So think about it like this: Syrian rebels (and their backers) are fighting the Syrian government (and their backers), and both sides are fighting IS. Although IS wants to overthrow the government, it also violently opposes the rebels.

Easy.

There's just one more piece of the (simplified) puzzle.

Introducing the Kurds

"The enemy of my enemy is my friend," is a saying that doesn't apply in Syria.

Kurdish people living in Syria's north declared the creation of an autonomous government in areas under their control in early 2014.

This added a fourth dimension to the widening conflict.

The Kurds - who say they support neither the government nor the opposition - have been battling IS along the Turkish border.

They have benefitted from considerable military support from the US, which sees them as one of most effective anti-IS forces on the ground.

Turkey is also fighting IS. But in this case, the enemy of their enemy is also their... enemy.

Basically, Turkey sees the Kurdish Popular Protection Units (YPG) militia in Syria as an extension of the outlawed Kurdistan Workers Party (PKK), which has been fighting the Turkish military for decades.

To prevent Syrian Kurds establishing a contiguous autonomous region along its border, Turkey has carried out air strikes on the YPG and backed a Syrian rebel offensive that drove IS out of the last bit of territory not controlled by the YPG.

Image copyrightAFP/Getty ImagesImage caption
Kurdish-led fighters supported by the US are battling both IS and Turkish-backed rebels

OK. But, hang on, you never explained why all these foreign powers are involved in the first place.

Russia has military interests in Syria, including its only Mediterranean naval base and an airbase in Latakia province. It intervened at the request of the Syrian government and said its motivation was to fight terrorism.

Analysts say it also wants to bolster its influence in the Middle East, and globally, by being the key player in Syria.

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Russia's intervention has helped to reverse the fortunes of the Assad government

Iran sees Mr Assad, a member of the heterodox Shia Alawite sect, as its closest Arab ally. Syria is also the main transit point for Iranian weapons shipments to Hezbollah in Lebanon.

The US began backing rebel groups because it said Mr Assad was responsible for widespread atrocities.

But in the last two years it been more focused on the battle against IS, and President Donald Trump suggested only days before Friday's missile strike that getting Mr Assad to leave power was no longer a priority for him.